There are many places where unwanted signal reflection (i.e., back-reflection) can occur in an optical communication system. Common places of signal reflection include optical fiber joins and optical fiber connectors and interfaces (e.g., at an optical receiver, where optical signals bombard the surface of a photodetector).
One possible byproduct of signal reflection is power loss (i.e., a reduction in optical signal strength as perceived by an optical receiver). Another possible byproduct is disruption of the light source (e.g., a light emitting diode (LED) or laser) that initiates optical communication. Disruption of a system's light source can lead to poor signal quality and noise. Yet another possible byproduct of signal reflection is ghost signals (i.e., spurious signals that are superimposed on an intended signal, or signal reflections that continue after an optical transmission has ceased). All of these byproducts (e.g., power loss, poor signal quality and ghost signals) increase the likelihood that an optical receiver will erroneously identify a false or non-existent signal, thereby corrupting a data transmission.
Back-reflections at an optical receiver are typically evaluated as “return loss”, which is the ratio of incident signal power to reflected signal power. It is not uncommon for an optical receiver's return loss to be on the order of 30%.
Some of the problems associated with signal reflection can be overcome by introducing an optical isolator near an optical link's transmitter. However, for low cost transmitters, the addition of an isolator can be costly. Furthermore, isolators do not prevent ghost signals from being detected at a link's receiver.
Other techniques for reducing the problems associated with optical signal reflection include the introduction of optical circulators or diffractive lens components.